Ion exchanged glass with high resistance to sharp contact failure and articles made therefrom

ABSTRACT

An article comprising an ion-exchanged glass material that prevents sharp contact flaws from entering a central region of the material that is under central tension and thus causing failure of the material. The glass material may be a glass or glass ceramic having a surface layer under compression. In some embodiments, the depth of the compressive layer is greater than about 75 μm. The greater depth of layer prevents flaws from penetrating the compressive layer to the region under tension.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/525,393, filed on Aug. 19,2011, the content of which is relied upon and incorporated herein byreference in its entirety.

BACKGROUND

The disclosure relates to articles made from strengthened glassmaterials. More particularly, the disclosure relates to articles madefrom strengthened glass materials that are resistant to damage caused bysharp contact flaws.

Glass materials, such as glasses and glass ceramics, are widely used inapplications such as viewing windows, touch screens, and externalhousings for various electronic devices. When used in these and otherapplications, glass materials are subject to impact, abrasion, and otherdamage-causing events that may lead to catastrophic failure of thematerial. To prevent such failure, the major surfaces of these glassmaterials are sometimes strengthened by either thermal or chemical meansto provide surface layers that are under compressive stress.

Compressive layers are most effective in preventing the propagation offlaws resulting from bending. However, failure analysis of such articlesindicate that bending stresses during failure of such devices during useis minimal.

SUMMARY

The present disclosure provides an ion-exchanged glass material thatprevents sharp contact flaws from entering a central region of thematerial that is under central tension, and articles made therefrom. Theglass material may be a glass or glass ceramic having a surface layerunder compression. In some embodiments, the depth of the compressivelayer is greater than about 75 μm. The greater depth of layer preventsflaws from penetrating the compressive layer to the central region undertension.

Accordingly, one aspect of the disclosure is to provide an articlecomprising a glass material. The glass material has a thickness of lessthan about 1.5 mm, a compressive layer extending from a surface of theglass material to a depth of layer of at least 75 microns (μm). Theglass material has an inner central region under a tension of up toabout 75 MPa, and a Vickers crack initiation threshold of at least about10 kgf, wherein the compressive layer is under a surface compressivestress of at least 250 MPa.

Another aspect of the disclosure is to provide a method of strengtheninga glass material. The method comprises: providing an ion exchangemedium, the ion exchange medium comprising a first cation having a firstconcentration and a second cation having a second concentration, whereinthe first concentration is greater than the second concentration, andwherein the first cation is larger than the second cation; providing aglass material comprising the second cation; and ion exchanging thesecond cations in the glass material with the first cations in the ionexchange medium, wherein ion exchanging the first cations with thesecond cations forms a layer under a compressive stress, the layerextending from a surface of the glass material to a depth of layer of atleast 75 μm, and wherein the glass material, after ion exchange, has aVickers crack initiation threshold of at least about 10 kgf.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of a strengthened sheet of aglass material;

FIG. 2 is a plot of the frangibility limit of a glass as a function ofthickness for glass samples;

FIG. 3 is a curve defining the frangibility limit of a glass, plotted asa function of compressive stress and depth of layer in FIG. 3 for athickness of 0.6 mm;

FIG. 4 is a plot of load to failure as a function of abrasion pressure;and

FIG. 5 is a plot of flaw depth as a function of abrasion pressure.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

Glass materials such as, but not limited to, glasses and glass ceramicsare widely used in applications such as viewing windows, touch screens,external housings, and the like for various electronic devices. Theseelectronic devices include portable communication and entertainmentdevices, laptop computers, televisions, and the like.

When used in these and other applications, glass materials are subjectto impact, abrasion, and other damage-causing events that may lead tocatastrophic failure of the material. To prevent such failure, the majorsurfaces of these glass materials are typically strengthened by eitherthermal or chemical means to provide surface layers that are undercompressive stress. In many instances, chemical strengthening of suchglass materials is achieved through ion exchange of the material.

In the ion exchange process, larger cations are introduced into thesurface layer of the glass and replace (or are “exchanged” with) smallercations having the same valence/charge/oxidation state. The increasedcation size in the glass network places the surface under compression;i.e., the replacement of smaller cations with larger cations forms alayer under compressive stress (compressive layer) that extends from thesurface of the glass material to a depth of layer (DOL). For example,potassium ions may be introduced into a sodium-containing glass, wherethe larger K⁺ cations replace smaller Na⁺ cations, thus forming acompressive layer at the surface of the glass. To balance or compensatefor the compressive stress at the surface, a tensile stress arises inthe inner or central region of the glass material.

Ion exchange of such glass materials is often achieved by immersing theglass material in an ion exchange bath comprising a salt of the largercation. The ion exchange bath is typically heated to a temperature atwhich the salt is molten, but below the strain and anneal points of theglass material. The ion exchange bath may also comprise other salts suchas, for example, salts of the cation that is removed from or replaced inthe glass material.

A cross-sectional schematic view of a strengthened sheet of glassmaterial is shown in FIG. 1. Article 100 has a thickness t, firstsurface 110, and second surface 112. Article 100, in some embodiments,has a thickness t of up to about 1.5 mm. While the embodiment shown inFIG. 1 depicts article 100 as a flat planar sheet or plate, article 100may have other configurations, such as three dimensional shapes ornon-planar configurations. Article 100 has a first compressive layer 120extending from first surface 110 to a depth of layer (DOL) d₁ into thebulk of the glass article 100. In the embodiment shown in FIG. 1,article 100 also has a second compressive layer 122 extending fromsecond surface 112 to a second depth of layer d₂. Article 100 also has acentral region 130 that extends from d₁ to d₂. Central region 130 isunder a tensile stress or central tension (CT), which balances orcounteracts the compressive stresses of layers 120 and 122. The depthd₁, d₂ of first and second compressive layers 120, 122 protects theglass article 100 from the propagation of flaws introduced by sharpimpact to first and second surfaces 110, 112 of glass article 100, whilethe compressive stress reduces the likelihood of a flaw penetratingthrough the depth d₁, d₂ of first and second compressive layers 120,122. Catastrophic failure of article 100 occurs when a flaw 140penetrates depth of layer d₁ into—and propagates through—central region130. Once flaw 140 surpasses depth of layer d₁, it continues to extendunder the tensile stress present in central region 130.

The magnitude of compressive stress within first and second compressivelayers 120, 122 has been believed to be critical to preventing failuresof such glass materials, particularly in the applications that have beenpreviously been described hereinabove. Compressive layers are mosteffective in preventing the propagation of flaws resulting from bending.As described herein, however, fractographic analyses of failures of sucharticles formed from strengthened glass materials indicate that bendingstresses during failure of such devices during use is minimal.

As described in detail in the instant disclosure, it has been discoveredthat failure of articles formed from such glass materials during useinstead occurs when the flaw depth exceeds the depth of layer; e.g.,flaw 140 (FIG. 1) extends beyond the depth d₁ of compressive layer 120into central region 130. Once the flaw depth penetrates the compressivelayer, the flaw enters the central region (130 in FIG. 1), which isunder central tension. The flaw will then typically continue topenetrate through the central region to depths that are several timesgreater than the depth of layer.

Examples that illustrate that failure of articles formed from such glassmaterials occurs when the flaw depth exceeds the depth of layer arelisted in Table 1. Table 1 lists flaw depths that caused in-use failureof articles comprising Corning GORILLA® Glass having a nominalcomposition of 69.2 mol % SiO₂, 8.5 mol % Al₂O₃, 13.9 mol % Na₂O, 1.2mol % K₂O, 6.5 mol % MgO, 0.5 mol % CaO, and 0.2 mol % SnO₂. All sampleslisted in Table 1 were ion exchanged to a depth of layer of about 50 μmand had a compressive stress of greater than about 650 MPa. As can beseen in Table 1, the measured depth of all flaws exceeded the 50 μmdepth of the compressive layer; i.e., failure occurred only when theflaw depth exceeded the depth of the compressive layer. At these flawdepths, failures occur with either with minimal amounts of externallyapplied stress or by slow extension of these flaws under the tensionpresent in the central region of the glass material.

TABLE 1 Depths of flaws that caused in use failure of glass articles.Sample Flaw Depth (μm) a 192 b 245 c 200 d 158 e 114 f 169 g 177 h 106 i90 j 52 k 278 l 454 m 86 n 284 o 90 p 73

Accordingly, a glass material that is resistant to sharp contactfailures and articles made therefrom are provided. The glass materialmay comprise as a glass, a glass ceramic, or the like. One aspect of thedisclosure is to provide an article comprising a strengthened glassmaterial having a surface layer under a compressive stress (compressivelayer) and a central region under tensile stress. The compressive layerextends from a surface of the article to a depth of layer, and thecentral region extends from the depth of layer into the bulk of theglass. The depth of the surface layer under compressive stress issufficient to prevent sharp contact flaws from penetrating or enteringthe central region under a tensile stress. In some embodiments, thedepth of layer is at least 70 μm and the compressive stress (CS) is atleast about 250 MPa. The glass material has a Vickers crack initiationthreshold, of at least about 5 kgf. In other embodiments, the glassmaterial has a Vickers crack initiation threshold of at least about 10kgf and, in still other embodiments, at least about 20 kgf. Vickersindentation cracking threshold measurements described herein areperformed by applying and then removing an indentation load to the glasssurface at 0.2 mm/min. The indentation maximum load is held for 10seconds. The indentation cracking threshold is defined at theindentation load at which 50% of 10 indents exhibit any number ofradial/median cracks emanating from the corners of the indentimpression. The maximum load is increased until the threshold is met fora given glass composition. All indentation measurements are performed atroom temperature in 50% relative humidity.

In another aspect, the strengthened glass materials described hereinhave a central region that is under a tensile stress (also referred toherein as “central tension” or “CT”). The relationship between CS and CTis given by the expression:

CT=(CS·DOL)/(t−2 DOL),

where t is the thickness of the glass article. In the above equation,central tension CT and compressive stress CS are expressed herein inmegaPascals (MPa), and the thickness t and depth of layer DOL areexpressed in millimeters. In the glass materials and articles madetherefrom described herein, the central tension is less than or equal tothe frangibility limit of the strengthened glass material. As usedherein, the “frangibility limit” refers to the central tension abovewhich the article comprising the glass material exhibits frangiblebehavior, and “frangibility” and “frangible behavior” refer to theenergetic fragmentation or crazing of a glass or glass material into alarge number of small pieces. As described in U.S. Published PatentApplication No. 2010/0009154 A1, the terms “frangible” and“frangibility” refer to the energetic fracture of a glass plate orsheet, when subjected to a point impact by an object or a drop onto asolid surface with sufficient force to break the glass plate intomultiple small pieces, with either multiple crack branching (i.e.,greater than 5 multiple cracks branching from an initial crack) in theglass, ejection of pieces from their original location of at least twoinches (about 5 cm), a fragmentation density of greater than about 5fragments/cm² of the glass plate, or any combination of these threeconditions. Conversely, a glass plate is deemed to be not frangible ifit either does not break or breaks with less than five multiple cracksbranching from an initial crack with pieces ejected less than two inchesfrom their original location when subjected to a point impact by anobject or a drop onto a solid surface with sufficient force to break theglass plate. Similarly, U.S. patent application Ser. No. 12/858,490describes frangible behavior as being characterized by at least one of:breaking of the strengthened glass article (e.g., a plate or sheet) intomultiple small pieces (e.g., ≦1 mm); the number of fragments formed perunit area of the glass article; multiple crack branching from an initialcrack in the glass article; and violent ejection of at least onefragment a specified distance (e.g., 5 cm, or about 2 inches) from itsoriginal location; and combinations of any of the foregoing breaking(size and density), cracking, and ejecting behaviors.

The frangibility limit of the glass material can be empiricallydetermined for glasses or glass materials of a given composition. FIG. 2is a plot of the frangibility limit (CT Limit in FIG. 2) as a functionof sample thickness for glass samples having a nominal composition of64.4 mol % SiO₂, 7.2 mol % B₂O₃, 13.9 mol % Al₂O₃, 14 mol % Na₂O, 0.5mol % K₂O, 0.1 mol % CaO, and 0.1 mol % SnO₂. The empirically determinedfrangibility limits may then be used to derive an overall expression forthe frangibility limit (line a in FIG. 3). The glass material shouldhave central tension that is less than or equal to the frangibilitylimit of the material. Accordingly, in some embodiments, thefrangibility limit of the articles and glass materials described hereinis less than about 75 MPa.

To illustrate the effect of depth of layer upon damage resistance, 0.6mm thick glass samples, each having a nominal composition of 64.4 mol %SiO₂, 7.2 mol % B₂O₃, 13.9 mol % Al₂O₃, 14 mol % Na₂O, 0.5 mol % K₂O,0.1 mol % CaO, and 0.1 mol % SnO₂, were ion exchanged to obtain threecombinations of compressive stress (CS) and depth of layer (DOL), whichwere selected along the curve defining the frangibility limit. The curvedefining the frangibility limit is plotted as a function of compressivestress and depth of layer for a thickness of 0.6 mm in FIG. 3. Samplesin group 1 had a depth of layer of 45 μm and a compressive stress of 744MPa. These samples were ion exchanged for 13 hours at 390° C. in a pure(100 wt %) potassium nitrate (KNO₃) ion exchange bath. Samples in group2 had a depth of layer of 72 μm and a compressive stress of 467 MPa.These samples were ion exchanged for 8 hours at 390° C. in an ionexchange bath comprising 94 wt % KNO₃ and 6% sodium nitrate (NaNO₃).Samples in group 3 had a depth of layer of 98 μm and a compressivestress of 345 MPa. These samples were ion exchanged for 24 hours at 390°C. in an ion exchange bath comprising 89 wt % KNO₃ and 11% sodiumnitrate (NaNO₃). Each sample was subjected to abrasion, followed bydetermination of the load to failure (i.e., crack formation originatingat the indent impression) using ring-on-ring testing for each sample.Abraded ring-on-ring failure loads obtained for glass samples weredetermined by first blasting the surface of the sample to be studied(dimensions are 50 mm×50 mm×0.6 mm thick) with 90 grit silicon carbide(SiC) at a pressure of 5 psi for five seconds. Samples are masked sothat the abrasion is limited to a 6 mm diameter circle located at thecenter of the 50 mm×50 mm faces of the sample. Abrasion of the sampleswas followed by ring-on-ring load to failure-testing with a 1 inchdiameter support ring and a ½ inch diameter loading ring. The sample isplaced on the support ring with the abraded side face down, so as to putthe abraded region in tension during testing. The load is applied at arate of 1.2 mm/min. Testing is performed at room temperature in 50%relative humidity. The radius of curvature on the rings is 1/16 inch.

Load to failure is plotted as a function of abrasion pressure is plottedin FIG. 4 for the samples described above. Damage becomes more severwith increasing abrasion pressure. Reduced damage resistance, which isindicated by the pressure at which the load to failure shiftssignificantly downward toward minimal values, begins to occur at 10 psifor those samples having 45 μm DOL (group 1 in FIG. 4). For thosesamples having a depth of layer of 72 μm (group 2 in FIG. 4), the onsetof reduced damage resistance occurs at 20 psi. An abrasion resistance ofgreater than 25 psi is required for reduced damage resistance to occurin those samples having a depth of layer of 98 μm (group 3 in FIG. 4).

Flaw depth is plotted as a function of abrasion pressure in FIG. 5 forthe samples described above. At 10 psi abrasion pressure, flaw depthexceeds the depth of layer for many samples having DOL of 45 μm (group1). At abrasion pressures greater than 10 kpsi, the flaw depth exceedsthe depth of layer for all samples having DOL of 45 μm. For thosesamples having a depth of layer of 72 μm (group 2), the flaw depthexceeds DOL at abrasion pressures of 20 and 25 psi. For those sampleshaving a depth of layer of 98 μm (group 3), the flaw depth exceeds DOLfor several samples at abrasion pressures of 25 psi.

The data plotted in FIG. 4 indicate that when the glass is subjected tomore severe contact (i.e., as abrasion pressure increases), the load tofailure remains high for glasses having greater depths of layer. FIG. 5shows that the drop off in load to failure corresponds to pressures thatpenetrate the depth of the compressive layer. For a given thickness andcentral tension, glasses having a compressive layer (i.e., greater DOL)are more resistant to sharp contact failure than those having a shallowcompressive layer.

Resistance to the formation of strength limiting flaws—i.e., medianand/or radial cracks—under wide angle contact such as by a Vickersindenter (136° 4-sided pyramidal indenter) improves with increasingcompressive stress CS at depths of layer greater than about 40 μm. Inthe case of wide angle Vickers indentation, the threshold for flawformation is much higher for glass with CS=849 MPa and DOL=43 μm, whencompared to glass with CS=366 MPa and DOL=100 μm. This is demonstratedin Table 1 by indentation with a Vickers (136°, 4-sided pyramidalindenter) indenter tip.

TABLE 1 Vickers indentation threshold load for the formation ofmedian/radial cracks for alkali aluminoborosilicate glasses withdifferent ion-exchange profiles. 0.6 mm glass alkali 0.6 mm alkalialuminoborosilicate glass aluminoborosilicate glass with CS = 849 DOL =43 with CS = 366 DOL = 100 Threshold (kgf) Threshold (kgf) 20-25 5-6

However, resistance to the formation of strength limiting flaws isgreatly reduced by contact with sharper indenters, such as, for example,a 100° 4-sided pyramidal indenter tip or by contact by irregularlyshaped 90 grit silicon carbide abrasive, as shown in Table 2. Sharpcontact promotes a displacive, shearing deformation mechanism thatgenerates greater subsurface damage and higher residual stress, thusleading to reduced threshold loads for median/radial cracking. Since theformation of flaws during sharp contact occurs at low contact loadsregardless of CS level, the resistance of central tension-driven glassfailure by complete separation of a glass plate into two of more piecesis achieved by containing the strength limiting flaws within the depthof the compressive layer. For example, 500 gf indentation with a 100°4-sided pyramidal indenter causes median/radial crack formation inglasses with either ion-exchange profile, CS=849 MPa and DOL=43 μm orCS=366 MPa and DOL=100 μm. Both ion-exchanged depths of layer aresufficient to prevent the flaw from 500 gf indentation from entering thecentral tension region. As the indentation load is increased to 750 gf,however, failure occurs in 50% of 10 indents made in the glass havingCS=849 MPa and DOL=43 μm. Parts that fail have indentation flaw depthsexceeding the depth of layer. At the 750 gf indentation load, none theparts with CS=366 MPa and DOL=100 μm fail, since the flaw depth does notexceed the DOL.

TABLE 2 Indentation of glasses with 100° 4-sided pyramidal indenter tip.Sharp indentation greatly reduces median/radial cracking threshold.Resistance to part failure increases for deeper DOL by containing theindentation flaws under compression. % of indents with % of indents thatcause % of indents with % of indents that cause Compressive Depth ofLayer median/radial failure by complete median/radial failure bycomplete Stress (MPa) (microns) cracks at 500 gf separation at 500 gfcracks at 750 gf separation at 750 gf 849 43 100 0 100 50 366 100 100 0100 0

In some embodiments, the glass material described herein is a glassceramic having a silicate glassy phase and a ceramic phase, and isstrengthened by ion exchange. Such glass ceramics include those in whichthe ceramic phase comprises or consists of β-spodumene, β-quartz,nepheline, kalsilite, carnegieite, combinations thereof, and the like.

In another embodiment, the alkali aluminosilicate glass comprises: fromabout 64 mol % to about 68 mol % SiO₂; from about 12 mol % to about 16mol % Na₂O; from about 8 mol % to about 12 mol % Al₂O₃; from 0 mol % toabout 3 mol % B₂O₃; from about 2 mol % to about 5 mol % K₂O; from about4 mol % to about 6 mol % MgO; and from 0 mol % to about 5 mol % CaO;wherein: 66 mol %≦SiO₂+B₂O₃+CaO≦69 mol %; Na₂O+K₂O+B₂O₃+MgO+CaO+SrO>10mol %; 5 mol %≦MgO+CaO+SrO≦8 mol %; (Na₂O+B₂O₃)−Al₂O₃≦2 mol %; 2 mol%≦Na₂O−Al₂O₃≦6 mol %; and 4 mol %≦(Na₂O+K₂O)−Al₂O₃≦10 mol %.

In another embodiment, the alkali aluminosilicate glass comprises: fromabout 60 mol % to about 70 mol % SiO₂; from about 6 mol % to about 14mol % Al₂O₃; from 0 mol % to about 15 mol % B₂O₃; from 0 mol % to about15 mol % Li₂O; from 0 mol % to about 20 mol % Na₂O; from 0 mol % toabout 10 mol % K₂O; from 0 mol % to about 8 mol % MgO; from 0 mol % toabout 10 mol % CaO; from 0 mol % to about 5 mol % ZrO₂; from 0 mol % toabout 1 mol % Sn0₂; from 0 mol % to about 1 mol % CeO₂; less than about50 ppm As₂O₃; and less than about 50 ppm Sb₂O₃; wherein 12 mol%≦Li₂O+Na₂O+K₂O≦20 mol % and 0 mol %≦MgO+CaO≦10 mol %.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂and Na₂O, wherein the glass has a temperature T_(35 kp) at which theglass has a viscosity of 35 kilo poise (kpoise), wherein the temperatureT_(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂ isgreater than T_(35 kp). In some embodiments, the alkali aluminosilicateglass comprises: from about 61 mol % to about 75 mol % SiO₂; from about7 mol % to about 15 mol % Al₂O₃; from 0 mol % to about 12 mol % B₂O₃;from about 9 mol % to about 21 mol % Na₂O; from 0 mol % to about 4 mol %K₂O; from 0 mol % to about 7 mol % MgO; and 0 mol % to about 3 mol %CaO.

In other embodiments, the alkali aluminosilicate glass comprises atleast 50 mol % SiO₂ and at least one modifier selected from the groupconsisting of alkali metal oxides and alkaline earth metal oxides,wherein [(Al₂O₃ (mol %)+B₂O₃ (mol %))/(Σ alkali metal modifiers (mol%))]>1. In some embodiments, the alkali aluminosilicate glass comprises:from 50 mol % to about 72 mol % SiO₂; from about 9 mol % to about 17 mol% Al₂O₃; from about 2 mol % to about 12 mol % B₂O₃; from about 8 mol %to about 16 mol % Na₂O; and from 0 mol % to about 4 mol % K₂O.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂,Al₂O₃, P₂O₅, and at least one alkali metal oxide (R₂O), wherein0.75≦[(P₂O₅ (mol %)+R₂O (mol %))/M₂O₃ (mol %)]≦1.3, whereM₂O₃=Al₂O₃+B₂O₃. In some embodiments, the alkali aluminosilicate glasscomprises: from about 40 mol % to about 70 mol % SiO₂; from 0 mol % toabout 28 mol % B₂O₃; from 0 mol % to about 28 mol % Al₂O₃; from about 1mol % to about 14 mol % P₂O₅; and from about 12 mol % to about 16 mol %R₂O; and, in certain embodiments, from about 40 to about 64 mol % SiO₂;from 0 mol % to about 8 mol % B₂O₃; from about 16 mol % to about 28 mol% Al₂O₃; from about 2 mol % to about 12% P₂O₅; and from about 12 mol %to about 16 mol % R₂O.

In still other embodiments, the alkali aluminosilicate glass comprisesat least about 5 mol % P₂O₅, wherein (M₂O₃ (mol %)/R_(x)O (mol %))<1,wherein M₂O₃=Al₂O₃+B₂O₃, and wherein R_(x)O is the sum of monovalent anddivalent cation oxides present in the alkali aluminosilicate glass. Insome embodiments, the monovalent and divalent cation oxides are selectedfrom the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO,BaO, and ZnO. In some embodiments, the glass comprises 0 mol % B₂O₃.

In still another embodiment, the alkali aluminosilicate glass comprisesat least about 50 mol % SiO₂ and at least about 11 mol % Na₂O, and thecompressive stress is at least about 900 MPa. In some embodiments, theglass further comprises Al₂O₃ and at least one of B₂O₃, K₂O, MgO andZnO, wherein−340+27.1·Al₂O₃−28.7·B₂O₃+15.6·Na₂O−61.4·K₂O+8.1·(MgO+ZnO)≧0 mol %. Inparticular embodiments, the glass comprises: from about 7 mol % to about26 mol % Al₂O₃; from 0 mol % to about 9 mol % B₂O₃; from about 11 mol %to about 25 mol % Na₂O; from 0 mol % to about 2.5 mol % K₂O; from 0 mol% to about 8.5 mol % MgO; and from 0 mol % to about 1.5 mol % CaO.

In some embodiments, the alkali aluminosilicate glasses described hereinare substantially free of (i.e., contain 0 mol % of) of at least one oflithium, boron, barium, strontium, bismuth, antimony, and arsenic.

In some embodiments, the alkali aluminosilicate glasses described hereinare down-drawable by processes known in the art, such as slot-drawing,fusion drawing, re-drawing, and the like, and has a liquidus viscosityof at least 130 kilopoise.

In another aspect, a method of strengthening a glass material, such asthose described hereinabove, is also provided. The method includesproviding an ion exchange medium that comprises a first cation and asecond cation. The first cation is present in the ion exchange medium ina first concentration and the second cation is present in the ionexchange medium in a second concentration, where the first concentrationis greater than the second concentration. The first cation is largerthan the second cation. In some embodiments, the ion exchange medium isan ion exchange bath such as, for example, a molten salt bath comprisingsalts of the first cation and, optionally, the second cation.

In some embodiments the method includes providing an ion exchangeableglass material comprising the second cation. The glass material may be aglass ceramic or an alkali aluminosilicate glass such as thosepreviously described hereinabove.

Second ions in the glass material are ion exchanged with first ions inthe ion exchange medium; i.e., first ions from the ion exchange mediumreplace second ions in the glass. In those embodiments in which the ionexchange medium is an ion exchange bath, ion exchange takes place byimmersing the glass material in the ion exchange bath. As a result ofthe ion exchange step, a layer under compressive stress (compressivelayer) extending from a surface of the glass material to a depth oflayer of at least 75 μm is formed and the ion exchanged glass materialhas a Vickers crack initiation threshold of at least about 10 kgf and,in some embodiments, at least 20 kgf.

In some embodiments, the first cation is the monovalent K⁺ cation andthe second cation is the monovalent Na⁺ cation. Here, the ion exchangebath may contain sodium and potassium salts. The ion exchange bath, forexample, may contain 89-94 wt % potassium nitrate and 6-11 wt % sodiumnitrate. The first and second cations may also be selected from theother alkali metal cations as well, provided that the first cation islarger than the second cation.

The ion exchange step may, in some embodiments, be carried out attemperatures ranging from about 390° C. up to about 450° C. for timesranging from about one hour up to about 24 hours and, in someembodiments, from about 8 hours up to about 24 hours.

The articles formed from the glass materials described herein may beused as viewing windows, touch screens, external housings, and the likefor various electronic devices. These electronic devices include, butare not limited to, portable communication and entertainment devices(e.g., telephones, music players, DVD players, etc.), laptop computers,televisions, and the like.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

1. An article comprising a glass material, the glass material having athickness of less than about 1.5 mm, a compressive layer extending froma surface of the glass material to a depth of layer of at least 75microns, an inner central region under a tension of up to about 75 MPa,and a Vickers crack initiation threshold of at least about 10 kgf,wherein the compressive layer is under a compressive stress of at least250 MPa.
 2. The article of claim 1, wherein the Vickers crack initiationthreshold is at least about 20 kgf.
 3. The article of claim 1, whereinthe glass material is strengthened by ion exchange.
 4. The article ofclaim 1, wherein the glass material is a glass ceramic comprising asilicate glassy phase and a ceramic phase.
 5. The article of claim 4,wherein the ceramic phase comprises at least one of β-spodumene,β-quartz, nepheline, kalsilite, carnegieite, and combinations thereof.6. The article of claim 1, wherein the glass material is an alkalialuminosilicate glass.
 7. The article of claim 6, wherein the alkalialuminosilicate glass comprising: from about 64 mol % to about 68 mol %SiO₂; from about 12 mol % to about 16 mol % Na₂O; from about 8 mol % toabout 12 mol % Al₂O₃; from 0 mol % to about 3 mol % B₂O₃; from about 2mol % to about 5 mol % K₂O; from about 4 mol % to about 6 mol % MgO; andfrom 0 mol % to about 5 mol % CaO; wherein: 66 mol %≦SiO₂+B₂O₃+CaO≦69mol %; Na₂O+K₂O+B₂O₃+MgO+CaO+SrO>10 mol %; 5 mol %≦MgO+CaO+SrO≦8 mol %;(Na₂O+B₂O₃)−Al₂O₃≦2 mol %; 2 mol %≦Na₂O−Al₂O₃≦6 mol %; and 4 mol%≦(Na₂O+K₂O)−Al₂O₃≦10 mol %.
 8. The article of claim 6, wherein thealkali aluminosilicate glass comprises: from about 60 mol % to about 70mol % SiO₂; from about 6 mol % to about 14 mol % Al₂O₃; from 0 mol % toabout 15 mol % B₂O₃; from 0 mol % to about 15 mol % Li₂O; from 0 mol %to about 20 mol % Na₂O; from 0 mol % to about 10 mol % K₂O; from 0 mol %to about 8 mol % MgO; from 0 mol % to about 10 mol % CaO; from 0 mol %to about 5 mol % ZrO₂; from 0 mol % to about 1 mol % SnO₂; from 0 mol %to about 1 mol % CeO₂; less than about 50 ppm As₂O₃; and less than about50 ppm Sb₂O₃; wherein 12 mol %≦Li₂O+Na₂O+K₂O≦20 mol % and 0 mol%≦MgO+CaO≦10 mol %.
 9. The article of claim 6, wherein the alkalialuminosilicate glass comprises SiO₂ and Na₂O, wherein the glass has atemperature T_(35 kp) at which the glass has a viscosity of 35 kpoise,wherein the temperature T_(breakdown) at which zircon breaks down toform ZrO₂ and SiO₂ is greater than T_(35 kp).
 10. The article of claim9, wherein the alkali aluminosilicate glass comprises: from about 61 mol% to about 75 mol % SiO₂; from about 7 mol % to about 15 mol % Al₂O₃;from 0 mol % to about 12 mol % B₂O₃; from about 9 mol % to about 21 mol%Na₂O; from 0 mol % to about 4 mol % K₂O; from 0 mol % to about 7 mol %MgO; and 0 mol % to about 3 mol % CaO.
 11. The article of claim 6,wherein the alkali aluminosilicate glass comprises at least 50 mol %SiO₂ and at least one modifier selected from the group consisting ofalkali metal oxides and alkaline earth metal oxides, wherein [(Al₂O₃(mol %)+B₂O₃ (mol %))/(Σ alkali metal modifiers (mol %))]>1.
 12. Thearticle of claim 11, wherein the alkali aluminosilicate glass comprises:from 50 mol % to about 72 mol % SiO₂; from about 9 mol % to about 17 mol% Al₂O₃; from about 2 mol % to about 12 mol % B₂O₃; from about 8 mol %to about 16 mol % Na₂O; and from 0 mol % to about 4 mol % K₂O.
 13. Thearticle of claim 6, wherein the alkali aluminosilicate glass comprisesSiO₂, Al₂O₃, P₂O₅, and at least one alkali metal oxide (R₂O), wherein0.75≦[(P₂O₅ (mol %)+R₂O (mol %))/M₂O₃ (mol %)]≦1.3, whereM₂O₃=Al₂O₃+B₂O₃.
 14. The article of claim 13, wherein the alkalialuminosilicate glass comprises: from about 40 mol % to about 70 mol %SiO₂; from 0 mol % to about 28 mol % B₂O₃; from 0 mol % to about 28 mol% Al₂O₃; from about 1 mol % to about 14 mol % P₂O₅; and from about 12mol % to about 16 mol % R₂O.
 15. The article of claim 14, wherein theglass comprises: from about 40 to about 64 mol % SiO₂; from 0 mol % toabout 8 mol % B₂O₃; from about 16 mol % to about 28 mol % Al₂O₃; fromabout 2 mol % to about 12% P₂O₅; and from about 12 mol % to about 16 mol% R₂O.
 16. The article of claim 6, wherein the alkali aluminosilicateglass comprises at least about 5 mol % P₂O₅, wherein (M₂O₃ (mol%)/R_(x)O (mol %))<1, wherein M₂O₃=Al₂O₃+B₂O₃, and wherein R_(x)O is thesum of monovalent and divalent cation oxides present in the alkalialuminosilicate glass.
 17. The article of claim 16, wherein themonovalent and divalent cation oxides are selected from the groupconsisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO, BaO, and ZnO.18. The article of claim 17, wherein the alkali aluminosilicate glasscomprises 0 mol % B₂O₃.
 19. The article of claim 6, wherein thecompressive stress is at least about 900 MPa and wherein the alkalialuminosilicate glass comprises at least about 50 mol % SiO₂ and atleast about 11 mol % Na₂O.
 20. The article of claim 19, wherein thealkali aluminosilicate glass further comprises Al₂O₃ and at least one ofB₂O₃, K₂O, MgO and ZnO, and wherein−340+27.1·Al₂O₃−28.7·B₂O₃+15.6·Na₂O−61.4·K₂O+8.1·(MgO+ZnO)≧0 mol %. 21.The article of claim 20, wherein the alkali aluminosilicate glasscomprises: from about 7 mol % to about 26 mol % Al₂O₃; from 0 mol % toabout 9 mol % B₂O₃; from about 11 mol % to about 25 mol % Na₂O; from 0mol % to about 2.5 mol % K₂O; from 0 mol % to about 8.5 mol % MgO; andfrom 0 mol % to about 1.5 mol % CaO.
 22. The article of claim 6, whereinthe alkali aluminosilicate glass has a liquidus viscosity of at leastabout130 kilopoise.
 23. The article of claim 1, wherein the glassmaterial is substantially free of oxides of at least one of lithium,boron, barium, strontium, bismuth, antimony, and arsenic.
 24. Thearticle of claim 1, wherein the glass material forms at least a portionof a viewing window, a touch screen, or an external housings for anelectronic device.
 25. A method of strengthening a glass material, themethod comprising: a. providing an ion exchange medium, the ion exchangemedium comprising a first cation having a first concentration and asecond cation having a second concentration, wherein the firstconcentration is greater than the second concentration, and wherein thefirst cation is larger than the second cation; b. providing a glassmaterial comprising the second cation; and c. ion exchanging the secondcations in the glass material with the first cations in the ion exchangemedium, wherein ion exchanging the first cation with the second cationforms a layer under a compressive stress, the layer extending from asurface of the glass material to a depth of layer of at least 75 μm, andwherein the glass material, after ion exchange, has a Vickers crackinitiation threshold of at least about 10 kgf.
 26. The method of claim25, wherein the ion exchange medium is an ion exchange bath, and whereinion exchanging the second cations in the glass material with firstcations in the ion exchange medium comprises immersing the glassmaterial in an ion exchange bath.
 27. The method of claim 26, whereinthe ion exchange bath is heated to a temperature between about 390° C.and about 450° C.
 28. The method of claim 26, wherein immersing theglass material in the ion exchange bath comprises immersing the glassmaterial for at least about eight hours.
 29. The method of claim 25,wherein the first cation is K⁺ and the second cation is Na⁺.
 30. Themethod of claim 25, wherein the glass material is one of a glass ceramicand an alkali aluminosilicate glass.
 31. The method of claim 25, whereinthe Vickers crack initiation threshold is at least about 20 kgf.